deflate

Inputs string into the deflate stream and returns the output from the stream. On calling this method, both the input and the output buffers of the stream are flushed. If string is nil, this method finishes the stream, just like Zlib::ZStream#finish.

The value of flush should be either Zlib::NO_FLUSH, Zlib::SYNC_FLUSH, Zlib::FULL_FLUSH, or Zlib::FINISH. See zlib.h for details.

TODO: document better!